A Search and Rescue system (SAR) is formed of one or more constellations of satellites which receive, on an uplink, an alerting signal coming from a radio beacon. This signal is transmitted on an international distress frequency. The alerting signal is retransmitted to a ground station responsible for extracting from it the distress information which is then sent to a mission control centre.
A known SAR system is the global Cospas-Sarsat system whose main application is the detection of accidents for boats, aircraft or individuals. The Cospas-Sarsat system notably uses a low-orbit constellation of satellites called LEOSAR (Low Earth Orbit Search and Rescue) for receiving alerting messages and transferring them to the ground station.
Another purpose of a SAR system is to locate the radio beacon transmitting the distress signal. For this purpose, the use of low-orbit satellites makes it possible to carry out location by the Doppler-Fizeau effect. A single satellite uses the arrival frequency of the alerting message data at several successive moments time-logged during its movement. As the arrival frequency of the received signal is different each time, it is therefore possible to derive the position of the radio beacon from it.
However, location by the Doppler-Fizeau effect has the major disadvantage of a long location time since a single satellite must carry out several successive measurements during its movement before being able to derive the position of the radio beacon from them. Moreover, the frequency measurements do not have sufficient accuracy for applications requiring very precise positioning. Finally, in order to have a sufficient relative speed, the Doppler-Fizeau measurement is principally usable for low-orbit satellites, which has certain disadvantages: the service life of satellites is shorter and the coverage rate for small sized constellations is also low (typically of the order of 35% for 6 satellites).
A development of the Cospas-Sarsat system consists in using a new constellation of satellites called MEOSAR (Medium Earth Orbit Search and Rescue) having a higher orbit. These satellites are positioned on an orbit principally used by GNSS (Global Navigation Satellite System) satellites such as the satellites of the GPS or GALILEO systems. This orbit is known as the Medium Earth Orbit (MEO) and corresponds to a region of space included between 2000 km and 35000 km. It is thus possible, with the same satellite, to benefit from the SAR alerting and the GNSS geolocation functions. This possibility is foreseen in the first generation of GALILEO satellites and for the third generation of GPS satellites in the years to come.
FIG. 1 is a diagrammatic representation of such a system in the case of application of GALILEO satellites. A radio beacon 101 communicates with a constellation of SAR satellites 102a,102b,102c,102d,102e. At least one of these satellites 102c is also a GNSS satellite. Some of these satellites can also have only the geolocation function. The radio beacon 101 transmits its distress information in an alerting message via an uplink 111,114 to a SAR satellite 102b,102c. The alerting message is then retransmitted to a ground station 104 via a downlink 112,115. The positioning of the radio beacon 101 is carried out principally by the use of a GNSS receiver in the beacon, the position thus calculated being retransmitted via the uplink between the beacon and the satellite. This receiver receives a positioning signal coming from at least four visible GNSS satellites and can derive its position therefrom by known means. The position is then transmitted on the uplink 111,115 with the distress message and arrives at the ground station 104 which can then communicate the position of the beacon to a control centre. An advantage of using medium-orbit satellites is that there is always at least one of these satellites visible from the ground, which makes it possible to ensure provision of an acknowledgement of reception of the alerting message by the ground station.
However, the use of a positioning receiver installed in the radio beacon has disadvantages related to the complexity of the processings to be carried out for the location and to the consumption of the beacon. In particular, in order to locate itself, the GNSS receiver must firstly carry out a search for at least four visible geolocation satellites. By way of example, the decoding of a GPS signal can take between 30 seconds and one minute for the calculation of a first point. The autonomy of the beacon is directly affected by this non-negligible processing time.
The present invention notably has an objective of reducing the complexity and the consumption of a radio beacon by using the functions of the alerting system for determining the positioning directly without using a GNSS receiver or by limiting its use. One of the objectives of the invention is also to reduce the lock-on time prior to determining the positioning. The joint use of the SAR system and the GNSS system is envisaged in order to exploit all of the available resources in an optimum manner.